Medication infusion device using negatively biased ambient pressure medication chamber

ABSTRACT

A method and apparatus for infusing medication into a patient&#39;s body using a medication chamber referenced to ambient pressure. The apparatus includes a medication chamber enclosed by a peripheral wall which includes a movable portion configured to transfer exterior ambient pressure into the chamber. Means are provided for exerting a negative bias force acting on the movable portion in a direction opposed to the ambient pressure force. Thus, the resultant pressure in the chamber will be negative with respect to ambient pressure, reducing the risk that the chamber can be overpressurized and produce an unintended medication discharge.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/294,973, filed Dec. 6, 2005, now U.S. Pat. No. 8,070,745, which is acontinuation of PCT Application Serial No. PCT/US2004/020117, filed Jun.23, 2004, which claims priority to U.S. Provisional Application Ser. No.60/483,015, filed Jun. 25, 2003.

FIELD OF THE INVENTION

This invention relates generally to medication infusion devices whichinclude a chamber for storing fluid medication and means for extractingmedication from the chamber for delivery to a patient's body site.

BACKGROUND OF THE INVENTION

Various types of implantable and/or external medication infusion devicesare described in the literature. For example only, see U.S. Pat. Nos.4,772,263 and 6,283,943 and the references cited therein which relateprimarily to implantable devices. Many such devices employ a medicationchamber together with a propellant reservoir which functions to isolatethe chamber from changes in ambient pressure attributable, for example,to changes in altitude. More particularly, a typical propellantreservoir contains a biphasic propellant balanced between gas and liquidphases to maintain a constant pressure regardless of changes inreservoir volume. The pressure in the medication chamber is typicallyreferenced (either positive or negative) to the constant reservoirpressure. Positive referenced devices have the advantage that thepropellant can be selected to provide a constant driving pressure underdefined operating conditions (e.g., constant flow applications) actingin a direction to force medication out of the chamber. Alternatively,negative referenced devices have inherent safety advantages; e.g., whenrefilling the chamber with a hypodermic needle, medication can be drawninto the chamber without the application of manual pressure to theneedle. This assures that the needle will not discharge medicationunless it has been properly placed in a device fill port and reduces thepossibility of chamber overpressurization. Also, during normaloperation, since chamber pressure is lower than ambient pressure, thepressure differential acts in a direction to draw fluid from the outletcatheter toward the chamber thus tending to reduce the risk ofmedication leakage into the patient's body.

Although the use of a propellant reservoir has the advantage ofisolating the medication chamber from changes in ambient pressure, itnevertheless adds to device size, complexity, and cost. Accordingly, ithas been recognized that, in some situations, it may be preferable toreference the medication chamber directly to ambient pressure. Forexample, U.S. Pat. No. 4,772,263 describes an infusion pump whichincludes a spring for producing a positive force on the drug chamber toforce the solution therefrom.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for infusingmedication into a patient's body using a medication chamber referencedto ambient pressure (so as to avoid the need for, and attendantcomplexity of, a propellant reservoir) while achieving the safety andreliability of negative referenced propellant reservoir designs.Embodiments of the invention can be configured for use either exteriorto a patient's body or implanted within a patient's body

An apparatus in accordance with the invention includes a medicationchamber enclosed by a peripheral (or boundary) member which includes amovable portion configured to transfer exterior ambient pressure intothe chamber. Means are provided in accordance with the invention forexerting a negative bias force acting on the movable portion in adirection opposed to the force produced by the ambient pressure. Thus,the resultant pressure in the chamber will always be negative withrespect to ambient pressure, reducing the risk that the chamber can beoverpressurized and produce an unintended medication discharge.

The peripheral member defining the chamber can be variously formed inaccordance with the invention. For example, the peripheral member (orwall) can be comprised of one or more rigid and/or flexible wallportions which cooperate to fully enclose the chamber. At least one wallportion is movable and has an exterior surface exposed to ambientpressure.

In one preferred embodiment, the peripheral member is defined by a rigidwall portion and a flexible wall portion, e.g., a resilient membrane,secured around its edge to the rigid wall portion to enclose the chambertherebetween. The exterior surface of the flexible wall portion isexposed to ambient pressure and a negative bias force is applied to theflexible wall portion acting in opposition to the ambient pressure. Thenegative bias force can be provided by various types of forcegenerators, e.g., a magnet, the inherent resiliency of a properlyconfigured resilient membrane, or by a spring member (e.g., leaf, coil,bellows, elastomeric material, etc). In any event, the bias force actsto create a pressure in the chamber which is negative with reference toambient.

In accordance with the invention, medication is extracted from thenegatively biased chamber by a selectively actuatable outlet pump.

In one alternative preferred embodiment, the chamber peripheral wallmember can be comprised of first and second rigid wall portionsconnected by a flexible wall portion, e.g., a flexible shroud orbellows, which permits the rigid wall portions to move toward and awayfrom one another to vary the chamber volume therebetween.

In a still further preferred embodiment, the chamber peripheral wall canbe formed by the interior wall surface of a hollow cylinder and by apiston mounted for reciprocal linear movement in the cylindrical volume.

Regardless of the particular implementation of the chamber peripheralwall, embodiments of the invention are characterized by a movable wallportion which is exposed to ambient pressure and a bias force acting inopposition to the ambient pressure to produce a resultant chamberpressure which is negative with respect to the ambient pressure. Thechamber peripheral wall, including the moveable wall portion, preferablyhas a geometry which optimizes volumetric efficiency, i.e., maximizesthe useable volume and minimizes dead space volume or ullage. The biasforce can be produced by a variety of force members including, forexample, discrete springs of various types, elastomeric material,magnets, etc.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic plan view of a preferred medication infusiondevice in accordance with the invention;

FIG. 2 is a schematic sectional view through the device of FIG. 1showing the movable portion (e.g., resilient membrane) of the chamberperipheral (or boundary) wall in a fully extended (i.e., chamber full)position;

FIG. 3 is a schematic representation of the movable chamber wall portiondepicting the application of ambient and bias forces to the wall portionin accordance with the invention;

FIGS. 4A and 4B schematically depict an alternative embodiment of theinvention using a spring to provide the bias and respectively showingthe movable wall portion in its compressed and extended positions;

FIG. 5 is a schematic illustration of a further alternative embodimentof the invention using a bellows or exterior spring to provide the biasforce;

FIG. 6 is a schematic illustration of a still further alternativeembodiment using magnetic repulsion to provide the bias force; and

FIG. 7 is a schematic illustration of a still further embodiment using ahollow cylinder and a movable piston to define the chamber.

DETAILED DESCRIPTION

Attention is now directed to FIGS. 1 and 2 which illustrate a preferredembodiment of a medication device 20 in accordance with the presentinvention for controllably delivering medication to a patients bodysite. Although the particular device 20 illustrated in FIGS. 1 and 2 isintended for implanting in a patient's body, it should be understoodthat the invention also finds utility in applications where the deviceis carried externally.

As depicted in FIGS. 1 and 2, the device 20 is comprised of a housing 24including a base plate 26 and a cover 28 supported on the base plate 26.The base plate 26 and cover 28 define one or more compartmentstherebetween, e.g., compartments 30, 34, 38 for housing variouscomponents such as battery 32, an electronics module 36, and an activemedication pump 40.

FIG. 2 depicts a flexible and resilient membrane 44 secured along itsedge 46 to the underside of the base plate 26. As will be explained ingreater detail hereinafter, the membrane 44 is configured to naturallyexpand to the fully extended position shown in FIG. 2 to maximize thespace, i.e., volume, of the closed medication chamber 50 formed betweenthe membrane 44 and base plate surface 47. When the content of chamber50 is evacuated, the ambient pressure acting against the membrane 44will collapse it against base plate surface 47.

An inlet valve 54 is supported by the cover 28 and base plate 26 andaffords communication to the interior of chamber 50. The inlet valve 54can be conventionally constructed comprising a self healing septum 56through which a hypodermic needle can be inserted to dischargemedication into the chamber 50. As will be discussed hereinafter,inasmuch as the medication chamber 50, in accordance with the presentinvention, is maintained at a negative pressure relative to ambientpressure, the hypodermic needle, when properly inserted through septum56, is able to discharge medication into the chamber 50 without theapplication of manual pressure to the hypodermic needle.

The active pump 40 has an inlet 60 which communicates with the chamber50 for extracting medication therefrom. The pump 40 is coupled to acatheter outlet connector 62 through which medication is pumped fordistribution to a body site.

In accordance with the present invention, the membrane 44 comprises amovable portion of a peripheral member or wall which defines andencloses the medication chamber 50. The exterior surface 68 of themembrane 44 is configured to be exposed to ambient pressure, i.e., thatis the internal body pressure when the implantable device 20 is in situ.Typically, this ambient pressure will be very close to atmosphericpressure, which of course is dependent upon altitude, temperature, etc.The ambient pressure acts in a direction tending to compress themembrane 44 against the base plate 26. More particularly, when thechamber 50 is filled with medication, the membrane 44 will expand to itsnatural fully extended position shown in FIG. 2. However, when themedication is evacuated by action of pump 40, then the ambient pressureacts to collapse the membrane 44 toward the base plate surface 47.

In accordance with the present invention, a spring bias force is appliedto the chamber movable wall portion, i.e., membrane 44 in FIG. 2, whichacts in a direction to oppose the ambient pressure force so as to createa residual pressure in the chamber which is negative with reference toambient.

More particularly, with reference to FIG. 3, note that the ambientpressure P_(A) acting on the exterior surface 68 of movable wall portion44 produces a force F_(A) tending to move the wall portion 44 toward thebase plate 26, i.e., to collapse the chamber 50. In accordance with thepresent invention, a bias force F_(B) is created which acts inopposition to the force F_(A). As shown, the chamber pressure P_(C) willbe negative with respect to the ambient pressure P_(A) attributable tothe negative bias force F_(B).

The force F_(B) can be provided in a variety of different ways. Forexample, the membrane 44 of FIG. 2 can comprise a part formed of metalor plastic material (e.g., nitinol, titanium, stainless steel, superalloys, composite material) configured so that in its natural orquiescent state it resiliently expands to the extended positionrepresented in FIGS. 2 and 3. Thus, as the ambient pressure bearsagainst, the movable wall portion of membrane 44 tending to move ittoward its compressed position, it will develop a restoration forceF_(B) acting to oppose the compression. As an alternative to configuringwall portion 44 to inherently exhibit the desired resilientcharacteristic, a separate force generator, e.g., a spring, a magnet, africtional member, etc. can be incorporated into the device structure.

Attention is now directed to FIG. 4A which depicts an alternativeembodiment 100. The embodiment 100 includes a base plate 102 and cover104 which can be considered identical to the corresponding components 26and 28 discussed in connection with FIG. 2. The plate 102 defines asubstantially rigid portion of a peripheral wall extending around andenclosing a medication chamber 106. The chamber peripheral wall, inaccordance with the present invention, also includes a movable portionwhich in embodiment 100 comprises a flexible boot or shroud 108. Theboot 108 carries a rigid wall portion 110 which is spaced from andoriented substantially parallel to plate 102. Thus, the chamber 106 inembodiment 100 is defined by the inner surfaces of wall portions 102 and110 and flexible wall portion or boot 108.

In the embodiment 100, a force generator, or member, comprises a coilspring 120 mounted between the inner surfaces of wall portions 102 and110. The spring member 120 is shown as a coil spring which is configuredso that in its natural or quiescent state, e.g., in a vacuum, it isextended to the position shown in FIG. 4B. Ambient pressure acting onthe outer surface of movable portion 110 acts in the direction tocompress spring member 120 with the spring member thus providing arestoration or bias force acting in opposition to the force of theambient pressure. Thus, the pressure within the chamber 106 will bemaintained below the ambient pressure as a consequence of the forceproduced by spring 120 as was discussed in connection with FIG. 3.Typically, this spring force is selected to produce a chamber pressurewhich is negative with respect to ambient pressure by a differentialwithin the range 0.1 to 5.0 psig.

FIG. 5 depicts a further alternative embodiment 200 in which the innersurfaces of a base plate 202, a movable rigid portion 204, and aflexible shroud or bellows 206 define and enclose a medication chamber208. A coil spring 210 is depicted as being formed around the exteriorof the shroud 206. The shroud 206 and coil spring 210 be formedseparately or alternatively can be formed as an integral bellows member.

The embodiment of FIG. 5 operates identically to the embodiment of FIGS.4A and 4B in that the spring 210 produces a bias force opposing theforce of the ambient pressure bearing on wall portion 204. As medicationis drawn from the chamber 208 by action of the active pump, the ambientpressure will displace wall portion 204 toward support plate 202 actingagainst the bias force provided by spring 210.

FIG. 6 illustrates an embodiment 300 which is similar in construction tothe embodiment 200 in FIG. 5. However, in lieu of using a spring memberto provide the negative bias force, the embodiment 300 uses magneticrepulsion to develop the negative bias force. More particularly, note inFIG. 6 that adjacent magnets 302 and 304 are similarly poled. Also notethat adjacent magnets 306 and 308 are similarly poled. Thus, as theforce produced by ambient pressure on the exterior surface of wallportion 310 acts to displace wall portion 310 toward base plate 312, therepulsion force produced by the magnets will increase in opposition tothe ambient force. Of course, as has been discussed in connection withthe earlier embodiments, this negative bias force will produce a chamberpressure which is negative with respect to the ambient pressure.

FIG. 7 depicts a still further embodiment 400. In the embodiment of FIG.7, a hollow cylinder 402 is provided defining an interior wall surface404. A piston 408 is mounted for reciprocal linear motion within thecylindrical volume defined by the interior wall surface 404. The piston408 interior surface 409, together with wall surface 404, defines amedication chamber 410. An inlet valve 412 opens to the medicationchamber and an outlet 414 couples the chamber 410 to an actuatable pump416. A force generator, e.g., spring member 418, is shown mounted in thechamber 410 bearing against piston interior surface 409. The pistonouter surface 420 is exposed to ambient pressure.

FIG. 7 depicts spring 418 in its expanded state with the chamber 410filled with medication supplied via inlet valve 412. As medication isextracted from the chamber 410 by action of the pump 416, ambientpressure acting on the piston outer surface 420 will act to move thepiston 408 along the interior wall surface 404 to compress spring 418and diminish the volume of chamber 410. This action will be opposed bythe restoration force of spring member 418 thus producing a pressure inchamber 410 which is negative with respect to the ambient pressureapplied to the piston surface 420.

From the foregoing, it should now be appreciated that multiple exemplaryembodiments have been described herein characterized by a chamberperipheral wall portion which is exposed to ambient pressure togetherwith means for producing a bias force acting in opposition to theambient pressure to produce a pressure within the chamber which isnegative with respect to the ambient pressure. Although only a limitednumber of embodiments have been specifically described, it should berecognized by those skilled in the art that the invention can beimplemented by a variety of alternative, essentially equivalent,structures conforming to the spirit of the invention and within theintended scope of the appended claims.

The invention claimed is:
 1. A method, comprising the steps of: biasing a movable wall member, which is carried by a pump housing and has a first side that defines a fluid chamber and a second side that is exposed to ambient pressure, away from the pump housing with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure; and pumping fluid out of the fluid chamber with an electrically actuated pump.
 2. A method as claimed in claim 1, further comprising the step of: implanting the movable wall member, pump housing and electrically actuated pump into a patient's body.
 3. A method as claimed in claim 1, wherein at least a portion of the wall member is resilient.
 4. A method as claimed in claim 1, wherein at least a portion of the wall member is a bellows.
 5. A method as claimed in claim 1, wherein the step of biasing a movable wall member comprises self-biasing a movable wall member away from the pump housing with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure.
 6. A method as claimed in claim 1, wherein the step of biasing a movable wall member comprises biasing a movable wall member away from the pump housing with a spring with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure.
 7. A method as claimed in claim 1, further comprising the step of: drawing fluid through a needle and into the fluid chamber with the pressure that is negative with respect to the ambient pressure.
 8. A method, comprising the step of: biasing a movable wall member, which is carried by a pump housing and has a first side that defines a fluid chamber and a second side that is exposed to ambient pressure, away from the pump housing with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure; and storing medication in the fluid chamber.
 9. A method as claimed in claim 8, further comprising the step of: pumping the medication out of the fluid chamber with a pump which is not the movable wall member that defines the fluid chamber.
 10. A method as claimed in claim 8, further comprising the step of: implanting the movable wall member and pump housing into a patient's body.
 11. A method as claimed in claim 8, wherein at least a portion of the wall member is resilient.
 12. A method as claimed in claim 8, wherein at least a portion of the wall member is a bellows.
 13. A method as claimed in claim 8, wherein the step of biasing a movable wall member comprises self-biasing a movable wall member away from the pump housing with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure.
 14. A method as claimed in claim 8, wherein the step of biasing a movable wall member comprises biasing a movable wall member away from the pump housing with a spring with enough force to create a pressure within the fluid chamber that is negative with respect to the ambient pressure.
 15. A method as claimed in claim 8, further comprising the step of: drawing fluid through a needle and into the fluid chamber with the pressure that is negative with respect to the ambient pressure. 